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.Dd May 22, 2018 .Dt HMAC 3 .Os .Sh NAME .Nm hmac .Nd compute a key-Hash Message Authentication Code .Sh LIBRARY .Lb libc .Sh SYNOPSIS n stdlib.h .Ft ssize_t .Fn hmac "const char *hname" "const void *key" "size_t klen" "const void *text" "size_t tlen" "void *digest" "size_t dlen" .Sh DESCRIPTION The .Fn hmac function computes the key-Hash Message Authentication Code per .Tn RFC 2104 and places the result in .Fa digest writing up to .Fa dlen bytes. The actual number of bytes that would be written is returned.
p The hash functions supported are: md2, md4, md5, rmd160, sha1, sha224, sha256, sha384, and sha512. .Sh RETURN VALUES The .Fn hmac function returns .Dv -1 if the .Fa hname is not found. Otherwise the actual length of the digest string is returned (which could be bigger or smaller than .Fa dlen ) . This length depends on the hashing function selected. .Sh SEE ALSO .Xr md2 3 , .Xr md4 3 , .Xr md5 3 , .Xr openssl_HMAC 3 , .Xr openssl_MD2 3 , .Xr openssl_MD4 3 , .Xr openssl_MD5 3 , .Xr rmd160 3 , .Xr sha1 3 , .Xr sha2 3 .Sh STANDARDS .Rs .%R RFC 2104 .Re .Sh NOTES The maximum digest length has been extended from .Dv 64 to .Dv 128 bytes to handle SHA2. .Sh HISTORY The .Fn hmac function appeared in .Nx 8 .
Indexes created Sat Sep 20 22:09:52 GMT 2025