---
name: "crypt"
section: "3"
platform: "macos"
package: "Libc"
version: "1752.120.2"
release: "macOS 26.5"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/crypt/3"
---

# crypt(3)

DES encryption

## NAME

**crypt**, **encrypt**, **setkey** – DES encryption

## SYNOPSIS

**#include \<unistd.h\>**

*char* *\**\

**crypt**(*const char \*key*, *const char \*salt*);

*void*\

**encrypt**(*char \*block*, *int edflag*);

**#include \<stdlib.h\>**

*void*\

**setkey**(*const char \*key*);

## DESCRIPTION

The **crypt**() function performs password encryption, based on the NBS Data Encryption Standard (DES). Additional code has been added to deter key search attempts. The first argument to **crypt**() is a null-terminated string, typically a user's typed password. The second is in one of two forms: if it begins with an underscore (\`\`\_''), an extended format is used in interpreting both the key and the salt value, as outlined below.

### Extended crypt:

The *key* is divided into groups of 8 characters (the last group is null-padded) and the low-order 7 bits of each each character (56 bits per group) are used to form the DES key as follows: the first group of 56 bits becomes the initial DES key. For each additional group, the XOR of the encryption of the current DES key with itself and the group bits becomes the next DES key.

The *salt* is a 9-character array consisting of an underscore, followed by 4 bytes of iteration count and 4 bytes of salt. These are encoded as printable characters, 6 bits per character, least significant character first. The values 0 to 63 are encoded as \`\`./0-9A-Za-z''. This allows 24 bits for both *count* and *salt*.

### Traditional crypt:

The first 8 bytes of the key are null-padded, and the low-order 7 bits of each character is used to form the 56-bit DES key.

The *salt* is a 2-character array of the ASCII-encoded salt. Thus, only 12 bits of salt are used. *count* is set to 25.

### Algorithm:

The *salt* introduces disorder in the DES algorithm in one of 16777216 or 4096 possible ways (ie. with 24 or 12 bits: if bit *i* of the *salt* is set, then bits *i* and *i+24* are swapped in the DES E-box output).

The DES key is used to encrypt a 64-bit constant, using *count* iterations of DES. The value returned is a null-terminated string, 20 or 13 bytes (plus null) in length, consisting of the *salt*, followed by the encoded 64-bit encryption.

The functions, **encrypt**() and **setkey**() provide access to the DES algorithm itself. **setkey**() is passed a 64-byte array of binary values (numeric 0 or 1). A 56-bit key is extracted from this array by dividing the array into groups of 8 and ignoring the last bit in each group. That bit is reserved for a byte parity check by DES, but is ignored by these functions.

The *block* argument to **encrypt**() is also a 64-byte array of binary values. If the value of *edflag* is 0, *block* is encrypted; otherwise, it is decrypted. The result is returned in the original array *block*, after using the key specified by **setkey**() to process it.

The function **crypt**() returns a pointer to the encrypted value on success, and NULL on failure.

The **crypt**() and **setkey**() functions all manipulate the same key space.

## SEE ALSO

[login(1)](/login/1.md), [passwd(1)](/passwd/1.md), [getpass(3)](/getpass/3.md), [compat(5)](/compat/5.md), [passwd(5)](/passwd/5.md)

## LEGACY SYNOPSIS

**#include \<unistd.h\>**

*int*\
 **encrypt**(*char \*block*, *int edflag*);

The function **encrypt**() returns 0 on success and 1 on failure.

*void*\
 **setkey**(*const char \*key*);

The include file ⟨unistd.h⟩ is necessary and sufficient for the **setkey**() function.

## BUGS

The **crypt**() function returns a pointer to static data, and subsequent calls to **crypt**() will modify the same object.

## HISTORY

A rotor-based **crypt**() function appeared in Version 6 AT&T UNIX. The current style **crypt**() first appeared in Version 7 AT&T UNIX.

This library (FreeSec 1.0) was developed outside the United States of America as an unencumbered replacement for the U.S.-only libcrypt encryption library. Programs linked against the  **crypt**() interface may be exported from the U.S.A. only if they use  **crypt**() solely for authentication purposes and avoid use of the other programmer interfaces listed above. Special care has been taken in the library so that programs which only use the  **crypt**() interface do not pull in the other components.

## AUTHOR

David Burren \<davidb@werj.com.au\>