Work with mRemoteNG passwords... Functionality has been rolled into https://github.com/realslacker/PSmRemoteNG

mRemoteNG-Password-Util.psm1

Add-Type -Path 'C:\Program Files (x86)\mRemoteNG\BouncyCastle.Crypto.dll'

function ConvertFrom-MRNGSecurePassword {

    param(

        [Parameter(Mandatory)]
        [ValidateNotNullOrEmpty()]
        [string]
        $EncryptedMessage,

        [Parameter(Mandatory)]
        [ValidateNotNullOrEmpty()]
        [securestring]
        $EncryptionKey

    )

    $Password = [System.Runtime.InteropServices.Marshal]::PtrToStringAuto( [System.Runtime.InteropServices.Marshal]::SecureStringToBSTR( $EncryptionKey ) )
    $EncryptedMessageBytes = [convert]::FromBase64String( $EncryptedMessage )

    $Engine = [Org.BouncyCastle.Crypto.Engines.AesEngine]::new()
    $Cipher = [Org.BouncyCastle.Crypto.Modes.GcmBlockCipher]::new($Engine)
    $Encoding = [System.Text.Encoding]::UTF8
    $SecureRandom = [Org.BouncyCastle.Security.SecureRandom]::new()
    
    $NonceBitSize = 128
    $MacBitSize   = 128
    $KeyBitSize   = 256
    $SaltBitSize  = 128
    $KeyDerivationIterations = 1000
    $MinPasswordLength = 1

    $Salt = New-Object byte[] ( $SaltBitSize/8 )
    [array]::Copy( $EncryptedMessageBytes, 0, $Salt,  0, $Salt.Length )

    Write-Verbose ( "Salt: {0}" -f ($Salt -join ',') ) -Verbose

    $PasswordInBytes = [Org.BouncyCastle.Crypto.PbeParametersGenerator]::Pkcs5PasswordToBytes( $Password.ToCharArray() )
    $KeyGenerator = [Org.BouncyCastle.Crypto.Generators.Pkcs5S2ParametersGenerator]::new()
    $KeyGenerator.Init( $PasswordInBytes, $Salt, $KeyDerivationIterations )
    $KeyParameter = $KeyGenerator.GenerateDerivedMacParameters($KeyBitSize)
    $KeyBytes = $KeyParameter.GetKey()

    $CipherStream = New-Object System.IO.MemoryStream (, $EncryptedMessageBytes )
    $CipherReader = New-Object System.IO.BinaryReader ( $CipherStream )

    $Payload = $CipherReader.ReadBytes( $Salt.Length )

    $Nonce = $CipherReader.ReadBytes( $NonceBitSize / 8 )

    Write-Verbose ( 'Nonce: {0}' -f ( $Nonce -join ',' ) ) -Verbose
    
    $Parameters = [Org.BouncyCastle.Crypto.Parameters.AeadParameters]::new( $KeyParameter, $MacBitSize, $Nonce, $Payload )

    $Cipher.Init( $false, $Parameters )

    $CipherTextBytes = $CipherReader.ReadBytes( $EncryptedMessageBytes.Length - $Nonce.Length )

    $PlainText = New-Object byte[] ( $Cipher.GetOutputSize( $CipherTextBytes.Length ) )

    $Len = $Cipher.ProcessBytes( $CipherTextBytes, 0, $CipherTextBytes.Length, $PlainText, 0 )
    $Cipher.DoFinal( $PlainText, $Len ) > $null

    $Encoding.GetString( $PlainText )

}

function ConvertTo-MRNGSecurePassword {

    param(

        [Parameter(Mandatory)]
        [ValidateNotNullOrEmpty()]
        [string]
        $Message,

        [Parameter(Mandatory)]
        [ValidateNotNullOrEmpty()]
        [securestring]
        $EncryptionKey

    )

    $Password = [System.Runtime.InteropServices.Marshal]::PtrToStringAuto( [System.Runtime.InteropServices.Marshal]::SecureStringToBSTR( $EncryptionKey ) )
    $MessageBytes = $Encoding.GetBytes($Message)

    $Engine = [Org.BouncyCastle.Crypto.Engines.AesEngine]::new()
    $Cipher = [Org.BouncyCastle.Crypto.Modes.GcmBlockCipher]::new($Engine)
    $Encoding = [System.Text.Encoding]::UTF8
    $SecureRandom = [Org.BouncyCastle.Security.SecureRandom]::new()

    $NonceBitSize = 128
    $MacBitSize   = 128
    $KeyBitSize   = 256
    $SaltBitSize  = 128
    $KeyDerivationIterations = 1000
    $MinPasswordLength = 1

    $Salt = New-Object byte[] ( $SaltBitSize/8 )
    $SecureRandom.NextBytes($Salt)

    Write-Verbose ( "Salt: {0}" -f ($Salt -join ',') ) -Verbose

    $PasswordInBytes = [Org.BouncyCastle.Crypto.PbeParametersGenerator]::Pkcs5PasswordToBytes( $Password.ToCharArray() )
    $KeyGenerator = [Org.BouncyCastle.Crypto.Generators.Pkcs5S2ParametersGenerator]::new()
    $KeyGenerator.Init( $PasswordInBytes, $Salt, $KeyDerivationIterations )
    $KeyParameter = $KeyGenerator.GenerateDerivedMacParameters($KeyBitSize)
    $KeyBytes = $KeyParameter.GetKey()

    $Payload = New-Object byte[] ( $Salt.Length )
    [array]::Copy( $Salt, 0, $Payload, 0, $Salt.Length )

    $Nonce = New-Object byte[] ( $NonceBitSize / 8 )
    $SecureRandom.NextBytes( $Nonce, 0, $Nonce.Length )

    Write-Verbose ( 'Nonce: {0}' -f ( $Nonce -join ',' ) ) -Verbose

    $Parameters = [Org.BouncyCastle.Crypto.Parameters.AeadParameters]::new( $KeyParameter, $MacBitSize, $Nonce, $Payload )

    $Cipher.Init( $true, $Parameters )

    $CipherText = New-Object byte[] ($Cipher.GetOutputSize( $MessageBytes.Length ))
    $Len = $Cipher.ProcessBytes( $MessageBytes, 0, $MessageBytes.Length, $CipherText, 0 )
    $Cipher.DoFinal( $CipherText, $Len ) > $null

    $CombinedStream = New-Object System.IO.MemoryStream
    $BinaryWriter = New-Object System.IO.BinaryWriter ( $CombinedStream )

    $BinaryWriter.Write( $Payload )
    $BinaryWriter.Write( $Nonce )
    $BinaryWriter.Write( $CipherText )

    [convert]::ToBase64String( $CombinedStream.ToArray() )

}

function New-MRNGRandomString ( $Length = 20 ) {

    $SecureRandom = [Org.BouncyCastle.Security.SecureRandom]::new()
    $StringBuilder = [System.Text.StringBuilder]::new()

    [char[]]$AvailableChars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789`~!@#$%^&*()-_=+|[]{};:',./<>?"

    for ( $i = 0; $i -lt $Length; $i ++ ) {

        $RandomIndex = $SecureRandom.Next( $AvailableChars.Length - 1 )
        $StringBuilder.Append( $AvailableChars[ $RandomIndex ] ) > $null

    }

    return $StringBuilder.ToString()

}

@realslacker Any idea what this could be?
I tried Powershell 7.0.1 but that did not solve the issue
I opened a issue on github for Powershell Core to see if they have any ideas:
PowerShell/PowerShell#12741

@realslacker It's been a while now and I thought to check in and see if maybe you found a solution to the issue?