Asymmetric Padding

Category: Utility · Node Type ID: padding-asymmetric

RSA OAEP (Encoding/Decoding) and RSA PSS (Encoding) padding schemes (PKCS#1 v2.2 / RFC 8017)

Overview

Performs standalone PKCS#1 v2.2 (RFC 8017) asymmetric message encoding and decoding using SHA-256 and MGF1-SHA256: EME-OAEP Encode (encryption padding), EME-OAEP Decode (decryption unpadding), and EMSA-PSS Encode (signature padding).

Both the visible parameters and the node's fixed input ports dynamically adapt to the selected Scheme:

In OAEP-Encode mode, it accepts Message and optional OAEP Label and produces a padded block of Target Length bytes ($128, 256, 384,$ or $512$).
In OAEP-Decode mode, it accepts Encoded (EM) and optional OAEP Label, verifies the OAEP structure, and recovers the original message.
In PSS-Encode mode, it accepts a 32-byte Message Hash ($\text{SHA-256}(M)$) and an optional explicit PSS Salt (or generates a random salt of PSS Salt Length bytes) to produce an EMSA-PSS encoded block ready for textbook RSA modular exponentiation.

When to Use

Inspecting the internal maskedDB / maskedSeed structure of RSA-OAEP or RSA-PSS blocks for educational analysis or CAVP test-vector verification (using an explicit PSS Salt).
Building custom raw RSA workflows separated into padding + modular exponentiation.

Parameter Configuration

Parameter (ID)	Type & Default	Visibility Condition	Description & Options
Scheme `scheme`	`select` Default: `OAEP-Encode`	Always visible	Selects the RFC 8017 operation: `OAEP-Encode`, `OAEP-Decode`, or `PSS-Encode`. Dynamically switches which input ports and properties are visible. `OAEP-Encode`: RSA-OAEP Encode (Encryption Padding) `OAEP-Decode`: RSA-OAEP Decode (Decryption Unpadding) `PSS-Encode`: RSA-PSS Encode (Signature Padding)
Modulus / Target Length (Bytes) `targetLength`	`select` Default: `256`	Always visible	RSA modulus byte length ($k \in \{128, 256, 384, 512\}$ corresponding to 1024, 2048, 3072, or 4096-bit RSA). Only visible when `Scheme` is `OAEP-Encode` or `PSS-Encode`. `128`: 128 Bytes (1024-bit RSA) `256`: 256 Bytes (2048-bit RSA) `384`: 384 Bytes (3072-bit RSA) `512`: 512 Bytes (4096-bit RSA)
PSS Salt Length (Bytes) `saltLength`	`number` Default: `32` Supports Variable Reference (`$var`: `int`)	When `scheme` is `PSS-Encode`	Length in bytes of the random salt generated when the optional `PSS Salt` input port is unconnected (default `32`). Only visible when `Scheme = PSS-Encode`.
Unify Errors `errorUnify`	`boolean` Default: `false` Supports Variable Reference (`$var`: `bool`)	Always visible	Replaces built-in errors with a custom error code/message when enabled.
Unified Error Code `errorUnifyCode`	`text` Default: `""` (empty) Supports Variable Reference (`$var`: `string`)	When `errorUnify` is `true`	Custom error code when Unify Errors is enabled.
Unified Error Message `errorUnifyMessage`	`text` Default: `""` (empty) Supports Variable Reference (`$var`: `string`)	When `errorUnify` is `true`	Custom error message when Unify Errors is enabled.

Port I/O Specification

See Ports & Connections for general wiring rules, data type colors, and dynamic port creation.

Input Ports

Port Nature: Fixed Input Ports (5 total; conditionally visible ports depend on parameter settings as shown below).

Port Name	Port ID	Data Type	Required	Visibility Condition	Description
`Message`	`message`	`Bytes`	Required	When `scheme` is `OAEP-Encode`	Raw message bytes to encode with EME-OAEP. Only visible when `Scheme = OAEP-Encode` (max length $k - 2 \times 32 - 2 = k - 66$ bytes).
`Encoded Message`	`em`	`Bytes` (EM)	Required	When `scheme` is `OAEP-Decode`	EME-OAEP encoded block (EM) to decode and unpad. Only visible when `Scheme = OAEP-Decode`.
`Message Hash`	`mHash`	`Bytes` (32B)	Required	When `scheme` is `PSS-Encode`	Pre-computed 32-byte SHA-256 hash of the message to encode with EMSA-PSS. Only visible when `Scheme = PSS-Encode`.
`OAEP Label`	`label`	`Bytes`	Optional	When `scheme` is `OAEP-Encode` or `OAEP-Decode`	Optional OAEP label bytes (hashed with SHA-256; defaults to empty bytes if unconnected). Only visible when `Scheme` is `OAEP-Encode` or `OAEP-Decode`.
`PSS Salt`	`salt`	`Bytes`	Optional	When `scheme` is `PSS-Encode`	Optional explicit PSS salt bytes for deterministic test vectors. If unconnected, a random salt of `PSS Salt Length` bytes is generated. Only visible when `Scheme = PSS-Encode`.

Output Ports

Port Nature: Fixed Output Ports (1 total).

Port Name	Port ID	Data Type	Visibility Condition	Description
`Result`	`result`	`Bytes`	Always visible

Error Cases

When any of the following conditions occurs at runtime, this node transitions to an ERROR state and downstream nodes connected to its outputs do not execute. Enabling Unify Errors (errorUnify) replaces all error codes below with your configured unified error code and message.

Error Code	Default Engine Message	Trigger Condition
`OAEP_MESSAGE_TOO_LONG`	Message is too long for OAEP	Raised in `OAEP-Encode` when `Message` exceeds $k - 66$ bytes (e.g., > 190 bytes for 2048-bit / 256-byte target length).
`INVALID_OAEP_PADDING`	Invalid OAEP padding	Raised when the encoded RSA-OAEP block fails structural or label-hash verification during decoding.
`INVALID_PSS_PARAMS`	Invalid PSS parameters (message hash must be 32 bytes, salt the same length as Salt Length, target length longer than hash and salt)	Raised when the message hash is not 32 bytes, the explicit salt length mismatches `PSS Salt Length`, or the modulus size is too small for the hash and salt.

Examples

Encoding and Decoding an RSA-OAEP Block

Pad a short message into a 256-byte (2048-bit) OAEP block and decode it back.

On the first Asymmetric Padding node, set Scheme to RSA-OAEP Encode and Target Length to 256 Bytes (2048-bit RSA), and connect your message to Message.
Connect its Encoded / Unpadded output to the Encoded (EM) input of a second Asymmetric Padding node configured with Scheme = RSA-OAEP Decode to recover the original message.