[cifs-protocol] [MS-SAMR] 3.2.2.5 Deriving an Encryption Key from a Plaintext Password

[Andreas Schneider]

On Thursday, July 14, 2022 10:03:02 AM CEST Andreas Schneider via cifs-
protocol wrote:
> Dear Dochelp Team,
> 
> I need your help again :-)
> 
> I'm trying to implement SamrUnicodeChangePasswordUser4. However when I try
> to run my implementation against Windows. I always get
> STATUS_WRONG_PASSWORD returned.
> 
> For the SamrUnicodeChangePasswordUser4 method (section 3.1.5.10.4), the
> shared secret is the plaintext old password and the CEK is generated as
> specified in section 3.2.2.5.
> 
> 3.2.2.5 Deriving an Encryption Key from a Plaintext Password
> 
> The client MUST derive the CEK in the following manner:
> CEK :: = (PBKDF2(NT HASH of “OldPassword”, Salt, IterationCount, 512))

I also guessed that 512 means that the PBKDF2 function uses SHA512 internally.

> Looking at the RFC 8018 section 5.2:
> 
> PBKDF2 (P, S, c, dkLen)
> 
>    Options:        PRF        underlying pseudorandom function (hLen
>                               denotes the length in octets of the
>                               pseudorandom function output)
> 
>    Input:          P          password, an octet string
>                    S          salt, an octet string
>                    c          iteration count, a positive integer
>                    dkLen      intended length in octets of the derived
>                               key, a positive integer, at most
>                               (2^32 - 1) * hLen
> 
>    Output:         DK         derived key, a dkLen-octet string
> 
> 
> The MS-SAMR document doesn't say a word about the dkLen. Which would be how
> many bytes the pbkdf2 function should return for the CEK.
> 
> I've used 16 bytes (same as the session key) as dkLen. However I get
> STATUS_WRONG_PASSWORD
> 
> 
> ./bin/rpcclient ncacn_np:earth.milkyway.site -U'bob%Pa$$w0rd at 3' -c
> 'chgpasswd4 bob Pa$$w0rd at 3 Pa$$w0rd at 6'
> [...]
> rpc_api_pipe: host earth.milkyway.site returned 4 bytes.
>      samr_ChangePasswordUser4: struct samr_ChangePasswordUser4
>         out: struct samr_ChangePasswordUser4
>             result                   : NT_STATUS_WRONG_PASSWORD
> 
> 
> I've uploaded traces to:
> 
> https://support.microsoft.com/files?
> workspace=eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiJ9.eyJ3c2lkIjoiNTY5YjBlMTItMzYyN
> S00NjhlLWIwNjgtOTBiZDYyZDk2MTllIiwic3IiOiIyMjA3MTEwMDQwMDA4ODMyIiwiYXBwaWQiO
> iI0ZTc2ODkxZC04NDUwLTRlNWUtYmUzOC1lYTNiZDZlZjIxZTUiLCJzdiI6InYxIiwicnMiOiJFe
> HRlcm5hbCIsInd0aWQiOiJhYzUxMDFlOS1mMTExLTQ5MGUtOGVlYS04NWMxNGMyNzMyNmIiLCJpc
> 3MiOiJodHRwczovL2FwaS5kdG1uZWJ1bGEubWljcm9zb2Z0LmNvbSIsImF1ZCI6Imh0dHA6Ly9zb
> WMiLCJleHAiOjE2NjU0MTQxMzEsIm5iZiI6MTY1NzYzODEzMX0.Oe0Nrl4WiClzTrLHTGeFVX6S-
> oHNH4LjSGoiVF9eXNo9wN9w-
> NyabVRaEUpWVvKheXcqukAuNYvxDGCnoj2ZbpPsE1JY4EByZfqC2l--8i6N0smD8Rtccd_YLg_hx
> 9SqGO- Dgr6Y5zLo6FMBUnfF6xQ8jhqB5a7ZJf4-
> TfMnCgXDsltrLzB_JU1rLDsVGI5ZzZfN9BEOJeKxS9PJEB3azUy8lFvcMsyq8ZL5LOzyQyhg7H2C
> glwDjzNeGmg2Wov8vdVdh3Ahk0AZ08Otf7i-7tpggx0F9FsH13oS2j6IOzEni23z2G6AqNL4j7ss
> _23sCp5njIL70rvGv3LliynERA&wid=569b0e12-3625-468e- b068-90bd62d9619e
> 
> 
> Help here would be much appreciated. Thanks you dochelp team.
> 
> 
> Best regards
> 
> 
> 	Andreas


-- 
Andreas Schneider                      asn at samba.org
Samba Team                             www.samba.org
GPG-ID:     8DFF53E18F2ABC8D8F3C92237EE0FC4DCC014E3D