Threat Model

The security of the data stored using sops is as strong as the weakest
cryptographic mechanism. Values are encrypted using AES256_GCM which is
the strongest symetric encryption algorithm known today. Data keys are
encrypted in either KMS, which also uses AES256_GCM, or PGP which uses
either RSA or ECDSA keys.

Going from the most likely to the least likely, the threats are as

Compromised AWS credentials grant access to KMS master key

An attacker with access to an AWS console can grant itself access to one
of the KMS master keys used to encrypt a sops data key. This threat
should be mitigated by protecting AWS accesses with strong controls,
such as multi-factor authentication, and also by performing regular
audits of permissions granted to AWS users.

Compromised PGP key

PGP keys are routinely mishandled, either because owners copy them from
machine to machine, or because the key is left forgotten on an unused
machine an attacker gains access to. When using PGP encryption, sops
users should take special care of PGP private keys, and store them on
smart cards or offline as often as possible.

Factorized RSA key

sops doesn't apply any restriction on the size or type of PGP keys. A
weak PGP keys, for example 512 bits RSA, could be factorized by an
attacker to gain access to the private key and decrypt the data key.
Users of sops should rely on strong keys, such as 2048+ bits RSA keys,
or 256+ bits ECDSA keys.

Weak AES cryptography

A vulnerability in AES256_GCM could potentially leak the data key or
the KMS master key used by a sops encrypted file. While no such
vulnerability exists today, we recommend that users keep their encrypted
files reasonably private.

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