BN_BN2BIN(3)

💖 NAME 💖

Welcome to BN_BN2BIN(3), bestie — this is the libcrypto page for the whole format-conversion squad: BN_bn2bin, BN_bn2binpad, BN_bin2bn, BN_bn2lebinpad, BN_lebin2bn, BN_bn2hex, BN_bn2dec, BN_hex2bn, BN_dec2bn, BN_asc2bn, BN_print, BN_print_fp, BN_bn2mpi, and BN_mpi2bn — they do format conversions, periodt.

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✨ SYNOPSIS ✨

Okay so the lineup is giving library “libcrypto” #include <openssl/bn.h> energy, and here are the exact signatures:

library “libcrypto” #include <openssl/bn.h>

int
BN_bn2bin(const BIGNUM *a, unsigned char *to);

int
BN_bn2binpad(const BIGNUM *a, unsigned char *to, int tolen);

BIGNUM *
BN_bin2bn(const unsigned char *s, int len, BIGNUM *ret);

int
BN_bn2lebinpad(const BIGNUM *a, unsigned char *to, int tolen);

BIGNUM *
BN_lebin2bn(const unsigned char *s, int len, BIGNUM *ret);

char *
BN_bn2hex(const BIGNUM *a);

char *
BN_bn2dec(const BIGNUM *a);

int
BN_hex2bn(BIGNUM **ap, const char *str);

int
BN_dec2bn(BIGNUM **ap, const char *str);

int
BN_asc2bn(BIGNUM **ap, const char *str);

int
BN_print(BIO *fp, const BIGNUM *a);

int
BN_print_fp(FILE *fp, const BIGNUM *a);

int
BN_bn2mpi(const BIGNUM *a, unsigned char *to);

BIGNUM *
BN_mpi2bn(unsigned char *s, int len, BIGNUM *ret);

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🦋 DESCRIPTION 🦋

• BN_bn2bin() — converts the absolute value of a into big-endian form and stores it at to. Bestie, to must point to BN_num_bytes(a) bytes of memory. That is not a suggestion, that is the assignment 💅.
• BN_bn2binpad() — also converts the absolute value of a into big-endian form and stores it at to. Here tolen indicates the length of the output buffer *to. The result is padded with zeros if necessary. If tolen is less than BN_num_bytes(a), an error is returned. So check your math before you commit, okay?
• BN_bin2bn() — converts the positive integer in big-endian form of length len at s into a BIGNUM and places it in ret. If ret is NULL, a new BIGNUM is created. New bestie acquired ✨.
• BN_bn2lebinpad() and BN_lebin2bn() — identical to BN_bn2binpad() and BN_bin2bn() except the buffer *to is in little-endian format. Little-endian girlies, this is your moment 🎀.
• BN_bn2hex() and BN_bn2dec() — return printable strings containing the hexadecimal and decimal encoding of a respectively. For negative numbers, the string is prefaced with a leading minus sign. The string must be freed later using free(3). Do not ghost that free call, bestie 😭.
• BN_hex2bn() — interprets str as a hexadecimal number. The string may start with a minus sign ('-'). Conversion stops at the first byte that is not a hexadecimal digit. The number is converted to a BIGNUM and stored in **ap. If *ap is NULL, a new BIGNUM is created. If ap is NULL, it only computes the number’s length in hexadecimal digits, also counting the leading minus sign if there is one. A "negative zero" is converted to zero. Okay, tiny detail but iconic.
• BN_dec2bn() — same as above but using the decimal system. Decimal girlies stay winning.
• BN_asc2bn() — infers the number base from an optional prefix. If str starts with "0x" or "0X", it calls BN_hex2bn(), otherwise BN_dec2bn(). If the number is negative, the minus sign can be given before or after the prefix. Flexible, lowkey.
• BN_print() and BN_print_fp() — write the hexadecimal encoding of a, with a leading minus sign for negative numbers, to the BIO or FILE fp. Print it and slay 🖨️.
• BN_bn2mpi() and BN_mpi2bn() — convert BIGNUMs from and to a format that consists of the number’s length in bytes represented as a 4-byte big-endian number, and the number itself in big-endian format, where the most significant bit signals a negative number (the representation of numbers with the MSB set is prefixed with a NUL byte). Very structured, very main character.
• BN_bn2mpi() — stores the representation of a at to, where *to must be large enough to hold the result. The size can be determined by calling BN_bn2mpi(a, NULL). That NULL call is the vibe check for your buffer size 🌈.
• BN_mpi2bn() — converts the len bytes long representation at s to a BIGNUM and stores it at ret, or in a newly allocated BIGNUM if ret is NULL. Cute and allocation-aware 🫶.

Bestie warning: the buffer-size requirements are real. Do not just hope for the best — that is not the move here 💀.

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👑 RETURN VALUES 👑

• BN_bn2bin() — returns the length of the big-endian number placed at to.
• BN_bn2binpad() and BN_bn2lebinpad() — return the number of bytes written or -1 if the supplied buffer is too small.
• BN_bin2bn() and BN_lebin2bn() — return the BIGNUM, or NULL on error.
• BN_bn2hex() and BN_bn2dec() — return a NUL-terminated string, or NULL on error.
• BN_hex2bn() and BN_dec2bn() — return the number’s length in hexadecimal or decimal digits, also counting the leading minus sign if there is one, or 0 on error, in which case no new BIGNUM is created. No new bestie on error, noted 📝.
• BN_asc2bn() — returns 1 on success or 0 on error, in which case no new BIGNUM is created.
• BN_print_fp() and BN_print() — return 1 on success, 0 on write errors.
• BN_bn2mpi() — returns the length of the representation.
• BN_mpi2bn() — returns the BIGNUM, or NULL on error.
• The error codes can be obtained by ERR_get_error(3).

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🌸 SEE ALSO 🌸

ASN1_INTEGER_to_BN(3), BN_new(3), BN_num_bytes(3), BN_zero(3)

Go read those too, bestie — cross-referencing is self-care 🧚.

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⭐ HISTORY ⭐

• BN_bn2bin(), BN_bin2bn(), and BN_print() first appeared in SSLeay 0.5.1.
• BN_print_fp() first appeared in SSLeay 0.6.0.
• BN_bn2hex(), BN_bn2dec(), BN_hex2bn(), BN_dec2bn(), BN_bn2mpi(), and BN_mpi2bn() first appeared in SSLeay 0.9.0.
• All these functions have been available since OpenBSD 2.4.
• BN_asc2bin() first appeared in OpenSSL 1.0.0 and has been available since OpenBSD 4.9. Yes, HISTORY literally spells it BN_asc2bin() — keeping that exactly as written, bestie 🫶.
• BN_bn2binpad(), BN_bn2lebinpad(), and BN_lebin2bn() first appeared in OpenSSL 1.1.0 and have been available since OpenBSD 7.0.

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Sign-off from the footer, because we keep the receipts: source/version is Debian, dated June 8, 2025, on the BN_BN2BIN(3) page — love u, bye bestie 💖🦋