239 lines
8.3 KiB
Groff
239 lines
8.3 KiB
Groff
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.\" ========================================================================
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.\"
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.IX Title "hmac 3"
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.TH hmac 3 "2018-03-27" "1.0.2o" "OpenSSL"
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.\" For nroff, turn off justification. Always turn off hyphenation; it makes
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.\" way too many mistakes in technical documents.
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.if n .ad l
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.nh
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.SH "NAME"
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HMAC, HMAC_CTX_init, HMAC_Init, HMAC_Init_ex, HMAC_Update, HMAC_Final, HMAC_CTX_cleanup,
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HMAC_cleanup \- HMAC message authentication code
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.SH "SYNOPSIS"
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.IX Header "SYNOPSIS"
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.Vb 1
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\& #include <openssl/hmac.h>
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\&
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\& unsigned char *HMAC(const EVP_MD *evp_md, const void *key,
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\& int key_len, const unsigned char *d, int n,
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\& unsigned char *md, unsigned int *md_len);
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\&
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\& void HMAC_CTX_init(HMAC_CTX *ctx);
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\&
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\& int HMAC_Init(HMAC_CTX *ctx, const void *key, int key_len,
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\& const EVP_MD *md);
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\& int HMAC_Init_ex(HMAC_CTX *ctx, const void *key, int key_len,
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\& const EVP_MD *md, ENGINE *impl);
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\& int HMAC_Update(HMAC_CTX *ctx, const unsigned char *data, int len);
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\& int HMAC_Final(HMAC_CTX *ctx, unsigned char *md, unsigned int *len);
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\&
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\& void HMAC_CTX_cleanup(HMAC_CTX *ctx);
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\& void HMAC_cleanup(HMAC_CTX *ctx);
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.Ve
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.SH "DESCRIPTION"
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.IX Header "DESCRIPTION"
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\&\s-1HMAC\s0 is a \s-1MAC \s0(message authentication code), i.e. a keyed hash
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function used for message authentication, which is based on a hash
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function.
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.PP
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\&\s-1\fIHMAC\s0()\fR computes the message authentication code of the \fBn\fR bytes at
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\&\fBd\fR using the hash function \fBevp_md\fR and the key \fBkey\fR which is
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\&\fBkey_len\fR bytes long.
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.PP
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It places the result in \fBmd\fR (which must have space for the output of
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the hash function, which is no more than \fB\s-1EVP_MAX_MD_SIZE\s0\fR bytes).
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If \fBmd\fR is \s-1NULL,\s0 the digest is placed in a static array. The size of
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the output is placed in \fBmd_len\fR, unless it is \fB\s-1NULL\s0\fR. Note: passing a \s-1NULL\s0
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value for \fBmd\fR to use the static array is not thread safe.
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.PP
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\&\fBevp_md\fR can be \fIEVP_sha1()\fR, \fIEVP_ripemd160()\fR etc.
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.PP
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\&\fIHMAC_CTX_init()\fR initialises a \fB\s-1HMAC_CTX\s0\fR before first use. It must be
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called.
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.PP
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\&\fIHMAC_CTX_cleanup()\fR erases the key and other data from the \fB\s-1HMAC_CTX\s0\fR
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and releases any associated resources. It must be called when an
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\&\fB\s-1HMAC_CTX\s0\fR is no longer required.
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.PP
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\&\fIHMAC_cleanup()\fR is an alias for \fIHMAC_CTX_cleanup()\fR included for back
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compatibility with 0.9.6b, it is deprecated.
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.PP
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The following functions may be used if the message is not completely
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stored in memory:
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.PP
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\&\fIHMAC_Init()\fR initializes a \fB\s-1HMAC_CTX\s0\fR structure to use the hash
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function \fBevp_md\fR and the key \fBkey\fR which is \fBkey_len\fR bytes
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long. It is deprecated and only included for backward compatibility
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with OpenSSL 0.9.6b.
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.PP
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\&\fIHMAC_Init_ex()\fR initializes or reuses a \fB\s-1HMAC_CTX\s0\fR structure to use the hash
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function \fBevp_md\fR and key \fBkey\fR. If both are \s-1NULL \s0(or \fBevp_md\fR is the same
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as the previous digest used by \fBctx\fR and \fBkey\fR is \s-1NULL\s0) the existing key is
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reused. \fBctx\fR must have been created with \fIHMAC_CTX_new()\fR before the first use
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of an \fB\s-1HMAC_CTX\s0\fR in this function. \fBN.B. \f(BIHMAC_Init()\fB had this undocumented
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behaviour in previous versions of OpenSSL \- failure to switch to \f(BIHMAC_Init_ex()\fB
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in programs that expect it will cause them to stop working\fR.
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.PP
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\&\fB\s-1NB:\s0 if \f(BIHMAC_Init_ex()\fB is called with \fBkey\fB \s-1NULL\s0 and \fBevp_md\fB is not the
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same as the previous digest used by \fBctx\fB then an error is returned
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because reuse of an existing key with a different digest is not supported.\fR
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.PP
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\&\fIHMAC_Update()\fR can be called repeatedly with chunks of the message to
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be authenticated (\fBlen\fR bytes at \fBdata\fR).
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.PP
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\&\fIHMAC_Final()\fR places the message authentication code in \fBmd\fR, which
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must have space for the hash function output.
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.SH "RETURN VALUES"
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.IX Header "RETURN VALUES"
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\&\s-1\fIHMAC\s0()\fR returns a pointer to the message authentication code or \s-1NULL\s0 if
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an error occurred.
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.PP
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\&\fIHMAC_Init_ex()\fR, \fIHMAC_Update()\fR and \fIHMAC_Final()\fR return 1 for success or 0 if
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an error occurred.
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.PP
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\&\fIHMAC_CTX_init()\fR and \fIHMAC_CTX_cleanup()\fR do not return values.
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.SH "CONFORMING TO"
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.IX Header "CONFORMING TO"
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\&\s-1RFC 2104\s0
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.SH "SEE ALSO"
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.IX Header "SEE ALSO"
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\&\fIsha\fR\|(3), \fIevp\fR\|(3)
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.SH "HISTORY"
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.IX Header "HISTORY"
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\&\s-1\fIHMAC\s0()\fR, \fIHMAC_Init()\fR, \fIHMAC_Update()\fR, \fIHMAC_Final()\fR and \fIHMAC_cleanup()\fR
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are available since SSLeay 0.9.0.
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.PP
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\&\fIHMAC_CTX_init()\fR, \fIHMAC_Init_ex()\fR and \fIHMAC_CTX_cleanup()\fR are available
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since OpenSSL 0.9.7.
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.PP
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\&\fIHMAC_Init_ex()\fR, \fIHMAC_Update()\fR and \fIHMAC_Final()\fR did not return values in
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versions of OpenSSL before 1.0.0.
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