Helpers

Constant-time test for equality

int sodium_memcmp(const void * const b1_, const void * const b2_, size_t len);

When a comparison involves secret data (e.g. a key, an authentication tag, etc), it is critical to use a constant-time comparison function. This property does not relate to computational complexity: it means the time needed to perform the comparison is the same for all data of the same size. The goal is to mitigate side-channel attacks.

The sodium_memcmp() function can be used for this purpose.

The function returns 0 if the len bytes pointed to by b1_ match the len bytes pointed to by b2_. Otherwise, it returns -1.

Note: sodium_memcmp() is not a lexicographic comparator and is not a generic replacement for memcmp().

Hexadecimal encoding/decoding

char *sodium_bin2hex(char * const hex, const size_t hex_maxlen,
                     const unsigned char * const bin, const size_t bin_len);

The sodium_bin2hex() function converts bin_len bytes stored at bin into a hexadecimal string.

The string is stored into hex and includes a nul byte (\0) terminator.

hex_maxlen is the maximum number of bytes that the function is allowed to write starting at hex. It must be at least bin_len * 2 + 1 bytes.

The function always returns hex. It evaluates in constant time for a given size.

int sodium_hex2bin(unsigned char * const bin, const size_t bin_maxlen,
                   const char * const hex, const size_t hex_len,
                   const char * const ignore, size_t * const bin_len,
                   const char ** const hex_end);

The sodium_hex2bin() function parses a hexadecimal string hex and converts it to a byte sequence.

hex does not have to be nul terminated, as the number of characters to parse is supplied via the hex_len parameter.

ignore is a string of characters to skip. For example, the string ": " allows colons and spaces to be present at any location in the hexadecimal string. These characters will just be ignored. As a result, "69:FC", "69 FC", "69 : FC" and "69FC" will be valid inputs and produce the same output.

ignore can be set to NULL to disallow any non-hexadecimal character.

bin_maxlen is the maximum number of bytes to put into bin.

The parser stops when a non-hexadecimal, non-ignored character is found or when bin_maxlen bytes have been written.

If hex_end is not NULL, it will be set to the address of the first byte after the last valid parsed character.

The function returns 0 on success.

It returns -1 if more than bin_maxlen bytes would be required to store the parsed string or the string couldn’t be fully parsed, but a valid pointer for hex_end was not provided.

It evaluates in constant time for a given length and format.

Base64 encoding/decoding

char *sodium_bin2base64(char * const b64, const size_t b64_maxlen,
                        const unsigned char * const bin, const size_t bin_len,
                        const int variant);

The sodium_bin2base64() function encodes bin as a Base64 string. variant must be one of:

  • sodium_base64_VARIANT_ORIGINAL

  • sodium_base64_VARIANT_ORIGINAL_NO_PADDING

  • sodium_base64_VARIANT_URLSAFE

  • sodium_base64_VARIANT_URLSAFE_NO_PADDING

None of these Base64 variants provides any form of encryption; just like hex encoding, anyone can decode them.

Computing a correct size for b64_maxlen is not straightforward and depends on the chosen variant.

The sodium_base64_ENCODED_LEN(BIN_LEN, VARIANT) macro returns the minimum number of bytes required to encode BIN_LEN bytes using the Base64 variant VARIANT. The returned length includes a trailing \0 byte.

The sodium_base64_encoded_len(size_t bin_len, int variant) function is also available for the same purpose.

int sodium_base642bin(unsigned char * const bin, const size_t bin_maxlen,
                      const char * const b64, const size_t b64_len,
                      const char * const ignore, size_t * const bin_len,
                      const char ** const b64_end, const int variant);

The sodium_base642bin() function decodes a Base64 string using the given variant and an optional set of characters to ignore (typically: whitespaces and newlines).

If b64_end is not NULL, it will be set to the address of the first byte after the last valid parsed character.

Base64 encodes 3 bytes as 4 characters, so the result of decoding a b64_len string will always be at most b64_len / 4 * 3 bytes long.

The function returns 0 on success.

It returns -1 if more than bin_maxlen bytes would be required to store the parsed string or the string couldn’t be fully parsed, but a valid pointer for b64_end was not provided.

Incrementing large numbers

void sodium_increment(unsigned char *n, const size_t nlen);

The sodium_increment() function takes a pointer to an arbitrary-long unsigned number and increments it.

It runs in constant time for a given length and considers the number to be encoded in a little-endian format.

sodium_increment() can be used to increment nonces in constant time.

Adding large numbers

void sodium_add(unsigned char *a, const unsigned char *b, const size_t len);

The sodium_add() function accepts two pointers to unsigned numbers encoded in little-endian format, a and b, both of size len bytes.

It computes (a + b) mod 2^(8*len) in constant time for a given length and overwrites a with the result.

Subtracting large numbers

void sodium_sub(unsigned char *a, const unsigned char *b, const size_t len);

The sodium_sub() function accepts two pointers to unsigned numbers encoded in little-endian format, a and b, both of size len bytes.

It computes (a - b) mod 2^(8*len) in constant time for a given length and overwrites a with the result.

This function was introduced in libsodium 1.0.17.

Comparing large numbers

int sodium_compare(const void * const b1_, const void * const b2_, size_t len);

Given b1_ and b2_, two len bytes numbers encoded in little-endian format, this function returns:

  • -1 if b1_ is less than b2_

  • 0 if b1_ equals b2_

  • 1 if b1_ is greater than b2_

The comparison is done in constant time for a given length.

This function can be used with nonces to prevent replay attacks.

Testing for all zeros

int sodium_is_zero(const unsigned char *n, const size_t nlen);

This function returns 1 if the nlen bytes vector pointed by n contains only zeros. It returns 0 if non-zero bits are found.

Its execution time is constant for a given length.

Clearing the stack

void sodium_stackzero(const size_t len);

The sodium_stackzero() function clears len bytes above the current stack pointer, to overwrite sensitive values that may have been temporarily stored on the stack.

Note that these values can still be present in registers.

This function was introduced in libsodium 1.0.16.

Notes

The sodium_base64_VARIANT_*() macros don’t have associated symbols. Bindings are encouraged to define specialized encoding/decoding functions instead.

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