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RSAREF(TM): A Cryptographic Toolkit for Privacy-Enhanced Mail Library Reference Manual RSA Laboratories March 2, 1992 Copyright (C) 1991-2 RSA Laboratories, a division of RSA Data Security, Inc. All rights reserved. INTRODUCTION This manual is a reference guide for users of RSAREF, a cryptographic toolkit designed to facilitate rapid development of Internet Privacy-Enhanced Mail (PEM) [1-3] implementations. RSAREF supports the following PEM-specified algorithms: - RSA encryption and key generation, as defined by RSA Data Security's Public-Key Cryptography Standards (PKCS) [4] - MD2 and MD5 message digests [3,5,6] - DES (Data Encryption Standard) in cipher-block chaining mode [7,8] RSAREF is written entirely in C. Its application interface includes the following routines: R_SignPEMBlock computes a digital signature on a message R_VerifyPEMSignature verifies a digital signature on a message R_VerifyBlockSignature verifies a digital signature on a block of data such as a certificate R_SealPEMBlock computes a digital signature and encrypts a message R_OpenPEMBlock decrypts an encrypted message and verifies a digital signature R_DigestBlock computes a message digest on a message R_GeneratePEMKeys generates an RSA public/private key pair R_RandomInit initializes a random structure R_RandomUpdate mixes bytes into a random structure R_GetRandomBytesNeeded computes the number of mix-in bytes still needed to seed a random structure R_RandomFinal zeroizes a random structure A simple PEM implementation can be built directly on top of these routines, together with message parsing and formatting routines and certificate-management routines. This manual is divided into five main parts and three appendices. This part introduces RSAREF. The next three parts explain RSAREF procedures: random structures; cryptographic enhancements; and key-pair generation. The last part documents the platform-specific run-time library. Appendix A lists RSAREF error types. Appendix B lists RSAREF types and constants. Appendix C lists platform-specific types and constants. RSAREF is intended to be compatible with forthcoming revised versions of the PEM RFCs, which are now in the unofficial "Internet-Draft" stage. RANDOM STRUCTURES A random structure contains a seed from which a pseudorandom sequence of bytes is derived. RSAREF generates keys and pads RSA encryption blocks with bytes derived from a random structure. Random structures are used by both message-processing and key-generation applications. RSAREF initializes a random structure with the procedure R_RandomInit. A typical application calls R_RandomInit on entry. A new random structure is not ready for use until it is seeded by mixing in some random bytes. RSAREF seeds a random structure with the procedure R_RandomUpdate and R_GetRandomBytesNeeded. A random structure is considered seeded when the number of bytes still needed reaches zero. More bytes can be mixed in after the random structure is seeded. A typical application calls R_GetRandomBytesNeeded and R_RandomUpdate immediately after calling R_RandomInit. RSAREF zeroizes a random structure with the procedure R_RandomFinal. A typical application calls R_RandomFinal on exit. R_RandomInit int R_RandomInit ( R_RANDOM_STRUCT *randomStruct /* new random structure */ ); R_RandomInit initializes a new random structure, storing the result in randomStruct. Return value: 0 success nonzero reserved for future compatibility R_RandomUpdate int R_RandomUpdate ( R_RANDOM_STRUCT *randomStruct, /* random structure */ unsigned char *block, /* block of values to mix in */ unsigned int blockLen /* length of block */ ); R_RandomUpdate mixes blockLen bytes from block into randomStruct. Return value: 0 success nonzero reserved for future compatibility R_GetRandomBytesNeeded int R_GetRandomBytesNeeded ( unsigned int *bytesNeeded, /* number of mix-in bytes needed */ R_RANDOM_STRUCT *randomStruct /* random structure */ ); R_GetRandomBytesNeeded computes the number of mix-in bytes still needed to seed randomStruct, storing the result in bytesNeeded. Return value: 0 success nonzero reserved for future compatibility R_RandomFinal void R_RandomFinal ( R_RANDOM_STRUCT *randomStruct /* random structure */ ); R_RandomFinal zeroizes randomStruct. No return value. CRYPTOGRAPHIC ENHANCEMENTS RSAREF applies cryptographic enhancements to messages with six procedures: R_SignPEMBlock, R_VerifyPEMSignature, R_VerifyBlockSignature, R_SealPEMBlock, R_OpenPEMBlock, and R_DigestBlock. The first five procedures are typically called by message-processing applications. R_DigestBlock is typically called by key-generation applications when computing the message digest of a prototype certificate [2,9]. To sign a message, an application calls R_SignPEMBlock, giving these arguments: - a pointer to the message content, and the message length - an integer identifying which message-digest algorithm to apply (MD2 or MD5) - a flag indicating whether to encode the message in printable ASCII according to RFC 1113 - the signer's RSA private key R_SignPEMBlock signs the message with the signer's private key and the specified message-digest algorithm, and optionally encodes the message in printable ASCII. It returns the signature, possibly the encoded message, and the status of the operation. The signature is encoded according to RFC 1113. To verify a signature on a message, an application calls R_VerifyPEMSignature, giving these arguments: - a pointer to the (possibly encoded) message, and the message length - a pointer to the signature, and the signature length - an integer identifying which message-digest algorithm was applied (MD2 or MD5) - a flag indicating whether the message was encoded in printable ASCII - the signer's RSA public key R_VerifyPEMSignature decodes the message if it was encoded and verifies the signature on the message with the signer's public key and the specified message-digest algorithm. It returns the message content if the message was encoded, and the status of the operation. To verify a signature on a block of data such as a certificate where the signature is not encoded in printable ASCII, an application calls R_VerifyBlockSignature, giving these arguments: - a pointer to the block, and the block length - a pointer to the signature, and the signature length - an integer identifying which message-digest algorithm was applied (MD2 or MD5) - the signer's RSA public key R_VerifyBlockSignature verifies the signature on the message with the signer's public key and the specified message-digest algorithm. It returns the status of the operation. To seal a message (sign and encrypt it), an application calls R_SealPEMBlock, giving these arguments: - a pointer to the message content, and the message length - an integer identifying which message-digest algorithm to apply (MD2 or MD5) - the signer's RSA private key - the recipient's RSA public key R_SealPEMBlock signs the message with the signer's private key and the specified message-digest algorithm, encrypts the message and the signature with a random DES key, and encrypts the DES key with the recipient's public key. It returns the encrypted message, the encrypted key, the encrypted signature, the DES initialization vector, and the status of the operation. The encrypted message, key, and signature are encoded according to RFC 1113. To open a message (decrypt it and verify its signature), an application calls R_OpenPEMBlock, giving these arguments: - a pointer to the encrypted message, and the encrypted message length - a pointer to the encrypted key, and the encrypted key length - a pointer to the encrypted signature, and the encrypted signature length - a DES initialization vector - an integer identifying which message-digest algorithm was applied (MD2 or MD5) - the signer's RSA public key - the recipient's RSA private key R_OpenPEMBlock decrypts the encrypted DES key with the recipient's private key, decrypts the encrypted message and the encrypted signature with the DES key, and verifies the signature on the message with the signer's public key and the specified message-digest algorithm. It returns the message content and the status of the operation. To digest a block of data such as a prototype certificate, an application calls R_DigestBlock, giving these arguments: - a pointer to the block, and the block length - an integer identifying which message-digest algorithm to apply (MD2 or MD5) R_DigestBlock digests the block with the specified message-digest algorithm. It returns the message digest and the status of the operation. ENCODED_CONTENT_LEN, DECODED_CONTENT_LEN, ENCRYPTED_CONTENT_LEN, and DECRYPTED_CONTENT_LEN are macros that assist in determining the maximum lengths of the results of cryptographic enhancements. R_SignPEMBlock int R_SignPEMBlock ( unsigned char *encodedContent, /* encoded content */ unsigned int *encodedContentLen, /* length of encoded content */ unsigned char *encodedSignature, /* encoded signature */ unsigned int *encodedSignatureLen, /* length of encoded signature */ unsigned char *content, /* content */ unsigned int contentLen, /* length of content */ int recode, /* recoding flag */ int digestAlgorithm, /* message-digest algorithm */ R_RSA_PRIVATE_KEY *privateKey /* signer's RSA private key */ ); R_SignPEMBlock computes a digital signature on content. Specifically, R_SignPEMBlock performs the following steps: 1. It digests content with digestAlgorithm, giving a message digest. 2. It encrypts the message digest with privateKey, giving a digital signature, and encodes the result in printable ASCII according to RFC 1113, storing the encoding in encodedSignature. 3. If recode is nonzero, it encodes content in printable ASCII, storing the encoding in encodedContent. If recode is nonzero, encodedContent will be an ASCII string, encoded according to RFC 1113. (It will not contain any line delimiters; the application must break the string into 64-character lines.) encodedContentLen will not be greater than ENCODED_CONTENT_LEN(contentLen). If recode is zero, encodedContent is ignored. encodedSignature will be an ASCII string, encoded according to RFC 1113. encodedSignatureLen will not be greater than MAX_PEM_SIGNATURE_LEN. digestAlgorithm is the algorithm by which the content is digested. It must be DA_MD2, which indicates the MD2 message-digest algorithm, or DA_MD5, which indicates the MD5 message-digest algorithm. Return value: 0 success RE_DIGEST_ALGORITHM digestAlgorithm is invalid RE_PRIVATE_KEY privateKey cannot encrypt message digest R_VerifyPEMSignature int R_VerifyPEMSignature ( unsigned char *content, /* content */ unsigned int *contentLen, /* length of content */ unsigned char *encodedContent, /* (possibly) encoded content */ unsigned int encodedContentLen, /* length of encoded content */ unsigned char *encodedSignature, /* encoded signature */ unsigned int encodedSignatureLen, /* length of encoded signature */ int recode, /* recoding flag */ int digestAlgorithm, /* message-digest algorithm */ R_RSA_PUBLIC_KEY *publicKey /* signer's RSA public key */ ); R_VerifyPEMSignature verifies a digital signature on a message. Its operation is the inverse of R_SignPEMBlock. R_VerifyPEMSignature operates on encodedSignature and encodedContent. If recode is nonzero, it first decodes encodedContent according to RFC 1113, and stores the result in content. If recode is zero, content is ignored. If recode is nonzero, contentLen will not be greater than DECODED_CONTENT_LEN(encodedContentLen). Return value: 0 success RE_CONTENT_ENCODING encodedContent has RFC 1113 encoding error RE_SIGNATURE_ENCODING encodedSignature has RFC 1113 encoding error RE_DIGEST_ALGORITHM digestAlgorithm is invalid RE_PUBLIC_KEY publicKey cannot decrypt signature RE_SIGNATURE signature on content is incorrect R_VerifyBlockSignature int R_VerifyBlockSignature ( unsigned char *block, /* block */ unsigned int blockLen, /* length of block */ unsigned char *signature, /* signature */ unsigned int signatureLen, /* length of signature */ int digestAlgorithm, /* message-digest algorithm */ R_RSA_PUBLIC_KEY *publicKey /* signer's RSA public key */ ); R_VerifyBlockSignature verifies a digital signature on a block of data such as a certificate. Its operation is similar to R_VerifyPEMSignature, except that the block and signature are arbitrary byte strings, rather than RFC 1113-encoded strings. Return value: 0 success RE_DIGEST_ALGORITHM digestAlgorithm is invalid RE_PUBLIC_KEY publicKey cannot decrypt signature RE_SIGNATURE signature on block is incorrect R_SealPEMBlock int R_SealPEMBlock ( unsigned char *encryptedContent, /* encoded, encrypted content */ unsigned int *encryptedContentLen, /* length of encoded, encrypted content */ unsigned char *encryptedKey, /* encoded, encrypted DES key */ unsigned int *encryptedKeyLen, /* length of encoded, encrypted DES key */ unsigned char *encryptedSignature,/* encoded, encrypted signature */ unsigned int *encryptedSignatureLen, /* length of encoded, encrypted signature */ unsigned char iv[8], /* DES initializing vector */ unsigned char *content, /* content */ unsigned int contentLen, /* length of content */ int digestAlgorithm, /* message-digest algorithm */ R_RSA_PUBLIC_KEY *publicKey, /* recipient's RSA public key */ R_RSA_PRIVATE_KEY *privateKey, /* signer's RSA private key */ R_RANDOM_STRUCT *randomStruct /* random structure */ ); R_SealPEMBlock computes a digital signature on content then encrypts the content and the signature. Specifically, R_SealPEMBlock performs the following steps: 1. It digests content with digestAlgorithm, giving a message digest. 2. It encrypts the message digest with privateKey, giving a digital signature. 3. It generates a random DES key and initializing vector, storing the initializing vector in iv. 4. It encrypts content with the DES key and initializing vector in cipher-block chaining mode, and encodes the result in printable ASCII according to RFC 1113, storing the encoding in encryptedContent. 5. It encrypts the DES key with publicKey and encodes the result in printable ASCII, storing the encoding in encryptedKey. 6. It encrypts the digital signature with the DES key and initializing vector, and encodes the result in printable ASCII, storing the encoding in encryptedSignature. encryptedContent will be an ASCII string, encoded according to RFC 1113. (It will not contain any line delimiters; the application must break the string into 64-character lines.) encryptedContentLen will not be greater than ENCRYPTED_CONTENT_LEN(contentLen). encryptedKey and encryptedSignature will be ASCII strings, encoded according to RFC 1113. encryptedKeyLen will not be greater than MAX_PEM_ENCRYPTED_KEY_LEN. encryptedSignatureLen will not be greater than MAX_PEM_ENCRYPTED_SIGNATURE_LEN. digestAlgorithm is the algorithm by which the content is digested. It must be DA_MD2, which indicates the MD2 message-digest algorithm, or DA_MD5, which indicates the MD5 message-digest algorithm. randomStruct must have been seeded. Return value: 0 success RE_DIGEST_ALGORITHM digestAlgorithm is invalid RE_PRIVATE_KEY privateKey cannot encrypt message digest RE_PUBLIC_KEY publicKey cannot encrypt DES key RE_NEED_RANDOM randomStruct is not seeded R_OpenPEMBlock int R_OpenPEMBlock ( unsigned char *content, /* content */ unsigned int *contentLen, /* length of content */ unsigned char *encryptedContent, /* encoded, encrypted content */ unsigned int encryptedContentLen, /* length of encoded, encrypted content */ unsigned char *encryptedKey, /* encoded, encrypted DES key */ unsigned int encryptedKeyLen, /* length of encoded, encrypted DES key */ unsigned char *encryptedSignature,/* encoded, encrypted signature */ unsigned int encryptedSignatureLen, /* length of encoded, encrypted signature */ unsigned char iv[8], /* DES initializing vector */ int digestAlgorithm, /* message-digest algorithm */ R_RSA_PRIVATE_KEY *privateKey, /* recipient's RSA private key */ R_RSA_PUBLIC_KEY *publicKey /* signer's RSA public key */ ); R_OpenPEMBlock decrypts an encrypted message and verifies a digital signature. Its operation is the inverse of R_SealPEMBlock. contentLen will not be greater than DECRYPTED_CONTENT_LEN(encryptedContentLen). Return value: 0 success RE_CONTENT_ENCODING encryptedContent has RFC 1113 encoding error RE_KEY_ENCODING encryptedKey has RFC 1113 encoding error RE_SIGNATURE_ENCODING encryptedSignature has RFC 1113 encoding error RE_PUBLIC_KEY publicKey cannot decrypt signature RE_PRIVATE_KEY privateKey cannot decrypt encrypted key RE_KEY recovered DES key cannot decrypt encrypted content or encrypted signature RE_DIGEST_ALGORITHM digestAlgorithm is invalid RE_SIGNATURE signature on content is incorrect R_DigestBlock int R_DigestBlock ( unsigned char *digest, /* message digest */ unsigned int *digestLen, /* length of message digest */ unsigned char *content, /* content */ unsigned int contentLen, /* length of content */ int digestAlgorithm /* message-digest algorithm */ ); R_DigestBlock computes a message digest on content, storing the resulting message digest in digest and its length in digestLen. digestAlgorithm is the algorithm by which the content is digested. It must be DA_MD2, which indicates the MD2 message-digest algorithm, or DA_MD5, which indicates the MD5 message-digest algorithm. digestLen will not be greater than MAX_DIGEST_LEN. Return value: 0 success RE_DIGEST_ALGORITHM digestAlgorithm is invalid KEY-PAIR GENERATION RSAREF generates key pairs with the procedure R_GeneratePEMKeys. R_GeneratePEMKeys is typically called by key generation applications. To generate a new key pair, an application calls R_GeneratePEMKeys, giving the length of the modulus, the choice of exponent (F4 or F0), and a random structure. R_GeneratePEMKeys generates an RSA key pair and returns the status of the operation. R_GeneratePEMKeys int R_GeneratePEMKeys ( R_RSA_PUBLIC_KEY *publicKey, /* new RSA public key */ R_RSA_PRIVATE_KEY *privateKey, /* new RSA private key */ R_RSA_PROTO_KEY *protoKey, /* RSA prototype key */ R_RANDOM_STRUCT *randomStruct /* random structure */ ); R_GeneratePEMKeys generates a random RSA key pair, storing the resulting RSA public key in publicKey and the resulting RSA private key in privateKey. Other parameters are as follows: protoKey The RSA prototype key specifying the length in bits of the RSA modulus and the public exponent. (See Appendix B.) randomStruct Random structure from which the key pair is derived. It must have been seeded. Return value: 0 success RE_MODULUS_LEN modulus length invalid RE_NEED_RANDOM randomStruct is not seeded RUN-TIME LIBRARY RSAREF operates on memory blocks with three platform-specific library procedures that are modeled after conventional C library functions: R_memcmp compares two blocks of memory R_memcpy copies a block of memory R_memset sets a block of memory to a given value These procedures can be found in the file 'r_stdlib.c'. R_memcmp int R_memcmp ( POINTER firstBlock, /* first block */ POINTER secondBlock, /* second block */ unsigned int len /* length of blocks */ ); R_memcmp compares the first len bytes of firstBlock and secondBlock. The value of len can be zero, in which case firstBlock and secondBlock are undefined and R_memcmp returns 0. R_memcmp compares the blocks by scanning the blocks from lowest address to highest until a difference is found. The smaller-valued block is the one with the smaller-valued byte at the point of difference. If no difference is found, the blocks are equal. Return value: < 0 firstBlock is smaller 0 blocks are equal > 0 firstBlock is larger R_memcpy void R_memcpy ( POINTER output, /* output block */ POINTER input, /* input block */ unsigned int len /* length of blocks */ ); R_memcpy copies the first len bytes of input to output. The value of len can be zero, in which output and input are undefined. The blocks do not overlap. No return value. R_memset void R_memset ( POINTER output, /* output block */ int value, /* value */ unsigned int len /* length of block */ ); R_memset sets the first len bytes of output to value. The value of len is zero, in which case output is undefined. No return value. APPENDIX A: RSAREF ERROR TYPES This appendix lists RSAREF's error types. RE_CONTENT_ENCODING content or encrypted content has RFC 1113 encoding error RE_DIGEST_ALGORITHM message-digest algorithm is invalid RE_KEY recovered DES key cannot decrypt encrypted content or encrypted signature RE_KEY_ENCODING encrypted key has RFC 1113 encoding error RE_MODULUS_LEN modulus length is invalid RE_NEED_RANDOM random structure is not seeded RE_PRIVATE_KEY private key cannot encrypt message digest, or cannot decrypt encrypted key RE_PUBLIC_KEY public key cannot encrypt DES key, or cannot decrypt signature RE_SIGNATURE signature on content or block is incorrect RE_SIGNATURE_ENCODING signature or encrypted signature has RFC 1113 encoding error APPENDIX B: RSAREF TYPES This appendix lists three RSAREF types: R_RSA_PUBLIC_KEY, R_RSA_PRIVATE_KEY, and R_RSA_PROTO_KEY. R_RSA_PUBLIC_KEY typedef struct { unsigned int bits; /* length in bits of modulus */ unsigned char modulus[MAX_RSA_MODULUS_LEN]; /* modulus */ unsigned char exponent[MAX_RSA_MODULUS_LEN]; /* public exponent */ } R_RSA_PUBLIC_KEY; An R_RSA_PUBLIC_KEY value is a structure specifying an RSA public key. There are three fields: bits length in bits of the modulus (not less than MIN_RSA_MODULUS_BITS and not greater than MAX_RSA_MODULUS_BITS) modulus modulus n, represented as a MAX_RSA_MODULUS_LEN-byte number, most significant byte first, as many leading zero bytes as necessary exponent public exponent e, represented like modulus R_RSA_PRIVATE_KEY typedef struct { unsigned int bits; /* length in bits of modulus */ unsigned char modulus[MAX_RSA_MODULUS_LEN]; /* modulus */ unsigned char publicExponent[MAX_RSA_MODULUS_LEN]; /* public exponent */ unsigned char exponent[MAX_RSA_MODULUS_LEN]; /* private exponent */ unsigned char prime[2][MAX_RSA_PRIME_LEN]; /* prime factors */ unsigned char primeExponent[2][MAX_RSA_PRIME_LEN]; /* exponents for CRT */ unsigned char coefficient[MAX_RSA_PRIME_LEN]; /* CRT coefficient */ } R_RSA_PRIVATE_KEY; An R_RSA_PRIVATE_KEY value is a structure specifying an RSA private key. There are seven fields: bits length in bits of the modulus (not less than MIN_RSA_MODULUS_BITS and not greater than MAX_RSA_MODULUS_BITS) modulus modulus n, represented as a MAX_RSA_MODULUS_LEN-byte number, most significant byte first, as many leading zero bytes as necessary publicExponent public exponent e, represented like modulus exponent private exponent d, represented like modulus prime prime factors p and q of modulus, each represented as MAX_RSA_PRIME_LEN-byte numbers, most significant byte first, as many leading zero bytes as necessary, where p > q primeExponents exponents (d mod p-1) and (d mod q-1) for Chinese remainder theorem (CRT) operations, each represented like prime factors coefficient coefficient (q^{-1} mod p) for Chinese remainder theorem operations, represented like prime factors R_RSA_PROTO_KEY /* RSA prototype key. */ typedef struct { unsigned int bits; /* length in bits of modulus */ int useFermat4; /* public exponent (1 = F4, 0 = 3) */ } R_RSA_PROTO_KEY; An R_RSA_PROTO_KEY value is a structure specifying the length in bits of the RSA modulus and the public exponent for key-pair generation. There are two fields: bits length in bits of the modulus (not less than MIN_RSA_MODULUS_BITS and not greater than MAX_RSA_MODULUS_BITS) useFermat4 a flag specifying the public exponent. If nonzero, it specifies F4 (65537); if 0, F0 (3) APPENDIX C: PLATFORM-SPECIFIC TYPES AND CONSTANTS This appendix lists three platform-specific types and one #define'd constant. TYPES RSAREF requires three platform-specific types: POINTER, UINT2, and UINT4. These are defined in the file 'global.h'. POINTER A POINTER value is a generic pointer to memory to which any other pointer can be cast. Example: typedef unsigned char *POINTER; UINT2 A UINT2 value is a 16-bit unsigned integer. Example: typedef unsigned short int UINT2; UINT4 A UINT4 value is a 32-bit unsigned integer. Example: typedef unsigned long int UINT4; #DEFINE'D CONSTANTS RSAREF requires one #define'd constant: PROTOTYPES. This is defined in the 'makefile' on the C compiler command line. PROTOTYPES indicates the form that C function declarations are to take. If PROTOTYPES is nonzero, declarations take the form type function (type, ..., type); Otherwise declarations take the form type function (); REFERENCES [1] J. Linn. RFC 1113: Privacy Enhancement for Internet Electronic Mail: Part I -- Message Encipherment and Authentication Procedures. Internet Activities Board, August 1989. [2] S. Kent and J. Linn. RFC 1114: Privacy Enhancement for Internet Electronic Mail: Part II -- Certificate-Based Key Management. Internet Activities Board, August 1989. [3] J. Linn. RFC 1115: Privacy Enhancement for Internet Electronic Mail: Part III -- Algorithms, Modes, and Identifiers. Internet Activities Board, August 1989. [4] RSA Data Security, Inc. PKCS #1: RSA Encryption Standard. Version 1.4, June 1991. Also published as NIST/OSI Implementors Workshop SEC-SIG-91-18, June 1991. (Available by anonymous FTP from 'rsa.com' as 'pub/pkcs/pkcs-1.ps'.) [5] B.S. Kaliski Jr. The MD2 Message-Digest Algorithm. July 1, 1991. (Available by anonymous FTP from 'rsa.com' as 'pub/md2.doc'.) [6] Ronald L. Rivest. The MD5 Message-Digest Algorithm. In preparation. (Earlier version available by anonymous FTP from 'rsa.com' as 'pub/md5.doc'.) [7] National Bureau of Standards. FIPS Publication 46-1: Data Encryption Standard. January 1988. [8] National Bureau of Standards. FIPS Publication 81: DES Modes of Operation. December 1980. [9] B.S. Kaliski Jr. Privacy Enhancement for Internet Electronic Mail: Part IV: Notary, Co-Issuer, CRL-Storing, and CRL-Retrieving Services. Draft, July 1, 1991. (Available by anonymous FTP from 'rsa.com' as 'pub/forms.doc'.)