MIT Kerberos Documentation

Keytab file format

There are two versions of the file format used by the FILE keytab type. The first byte of the file always has the value 5, and the value of the second byte contains the version number (1 or 2). Version 1 of the file format uses native byte order for integer representations. Version 2 always uses big-endian byte order.

After the two-byte version indicator, the file contains a sequence of signed 32-bit record lengths followed by key records or holes. A positive record length indicates a valid key entry whose size is equal to or less than the record length. A negative length indicates a zero-filled hole whose size is the inverse of the length. A length of 0 indicates the end of the file.

Key entry format

A key entry may be smaller in size than the record length which precedes it, because it may have replaced a hole which is larger than the key entry. Key entries use the following informal grammar:

entry ::=
    timestamp (32 bits)
    key version (8 bits)
    enctype (16 bits)
    key length (32 bits)
    key contents

principal ::=
    count of components (32 bits) [includes realm in version 1]
    realm (data)
    component1 (data)
    component2 (data)
    name type (32 bits) [omitted in version 1]

data ::=
    length (16 bits)
    value (length bytes)

Some implementations of Kerberos recognize a 32-bit key version at the end of an entry, if the record length is at least 4 bytes longer than the entry and the value of those 32 bits is not 0. If present, this key version supersedes the 8-bit key version. MIT krb5 does not yet implement this extension.