DPAPIinterface

DPAPIinterface allows R users on Windows OS to use Microsoft's DPAPI to encrypt and decrypt data. The code is connecting DPAPI functions and R are written in C++ and therefore depends on the Rcpp library.

Encryption

DPAPIinterface::encrypt(str, entropy = "")

• str argument is a Character Vector of length 1
• entropy argument is a Character Vector of length 1
• returns a numeric vector of arbitrary length

Decryption

DPAPIinterface::decrypt(vec, entropy = "")

• vec argument is a numeric vector
• entropy argument is a Character Vector of length 1
• returns a character vector of length 1

Example

Dramatis Personae:

• Bob, an R programmer
• Alice, another R programmer
• Eve, a nefarious actor

Data can be encrypted without secondary entropy, which is used to derive an additional key necessary for decryption. For example, Bob may encrypt a string without secondary entropy

cryptic <- DPAPIinterface::encrypt("not aPassword123!!!")
notcryptic <- DPAPIinterface::decrypt(cryptic)
cryptic
notcryptic

Eve would only be able to decrypt cryptic if she either knows Bob's Windows user password at the time of encryption, or gains sufficient access Bob's system to run a DPAPI function under Bob's account.

Alice is worried that her system may be compromised at some point of time, so she uses "mellon" for her secondary entropy:

cryptic2 <- DPAPIinterface::encrypt("Khazad-dum", "mellon")
cryptic2

(In production code, it's probably better to prompt for secondary entropy each session instead of hard coding it)

Alice knows what she used for secondary entropy, so she can decrypt cryptic2

DPAPIinterface::decrypt(cryptic2, "mellon")

Eve doesn't know what string Alice used for secondary entropy, so she can't decrypt even if she manages to execute DPAPI functions under Alice's account

DPAPIinterface::decrypt(cryptic2)

Under the hood

• encrypt converts an R character vector of length 1 to a C++ std::string, which is fed to CryptProtectData after a bit of manipulation. entropy is processed similarly. The output DATA_BLOB is converted to a string that is then converted to an R numeric vector to circummvent issues with translating arbitrary C++ std::string to R character vector
• decrypt reverses the steps of encrypt
• entropy provides secondary entropy, which in my understanding modifies the encryption key without changing the difficulty of brute-force attacks

To-do

• Change how int array (of bytes) to string conversion works: allocate placeholder string of correct length first instead of concatenating one letter at a time
• Check platform before installation and give suitable error messages for non-Windows OS
• Actual manual