decrypt-data.md

ID	C0031
Objective(s)	Cryptography
Related ATT&CK Techniques	None
Version	2.0
Created	13 October 2020
Last Modified	13 September 2023

Decrypt Data

Malware may decrypt data.

Methods

Name	ID	Description
AES	C0031.001	Malware decrypts data encrypted with the AES algorithm.
Block Cipher	C0031.002	Malware decrypts data encrypted with a block cipher.
Blowfish	C0031.003	Malware decrypts data encrypted with the Blowfish algorithm.
Camellia	C0031.004	Malware decrypts data encrypted with the Camellia algorithm.
3DES	C0031.005	Malware decrypts data encrypted with the 3DES algorithm.
HC-128	C0031.006	Malware decrypts data encrypted with the HC-128 algorithm.
HC-256	C0031.007	Malware decrypts data encrypted with the HC-256 algorithm.
RC4	C0031.008	Malware decrypts data encrypted with the RC4 algorithm.
RC6	C0031.009	Malware decrypts data encrypted with the RC6 algorithm.
RSA	C0031.010	Malware decrypts data encrypted with the RSA algorithm.
Skipjack	C0031.011	Malware decrypts data encrypted with the Skipjack block cipher algorithm.
Sosemanuk	C0031.012	Malware decrypts data encrypted with the Sosemanuk stream cipher.
Stream Cipher	C0031.013	Malware decrypts data encrypted with a stream cipher.
Twofish	C0031.014	Malware decrypts data encrypted with the Twofish algorithm.

Use in Malware

Name	Date	Method	Description
BlackEnergy	2007	--	BlackEnergy encrypts or decrypts via WinCrypt. [1]
Kovter	2016	--	Encrypt or decrypt via WinCrypt (This capa rule had 1 match) [1]

Detection

Tool: capa	Mapping	APIs
encrypt or decrypt via WinCrypt	Decrypt Data (C0031)	CryptEncrypt, CryptDecrypt, CryptAcquireContext, CryptGenKey, CryptImportKey
decrypt data using AES via x86 extensions	Decrypt Data::AES (C0031.001)

Code Snippets

C0031 Snippet

Decrypt Data

SHA256: c86cbf5e78c9f05ecfc11e4f2c147781cef77842a457e19ba690477eb564c22b

asm
push    ebx
mov     ebx, [esp+4+arg_4]
push    esi
lea     eax, [ebx+20h]
push    eax             ; unsigned int
call    ??2@YAPAXI@Z    ; operator new(uint)
mov     ecx, [esp+0Ch+arg_C]
mov     edx, eax
add     esp, 4
mov     esi, [ecx]
mov     [edx], esi
mov     esi, [ecx+4]
mov     [edx+4], esi
mov     ecx, [ecx+8]
mov     [edx+8], ecx
mov     edx, [esp+8+arg_8]
test    ebx, ebx
mov     [eax+0Ch], edx
jle     short loc_B
mov     esi, [esp+8+arg_0]
push    edi
mov     edi, 0FFFFFFFDh
lea     edx, [eax+3]
sub     edi, eax
loc_A:
mov     cl, [edx-3]
xor     cl, [edx+2]
xor     cl, [edx-1]
xor     cl, [edx]
mov     [edx+0Dh], cl
xor     [esi], cl
inc     edx
inc     esi
lea     ecx, [edi+edx]
cmp     ecx, ebx
jl      short loc_A
pop     edi
loc_B:
push    eax             ; void *
call    ??3@YAXPAX@Z    ; operator delete(void *)
add     esp, 4
mov     eax, 1
pop     esi
pop     ebx
retn

References

[1] capa v4.0, analyzed at MITRE on 10/12/2022