It just got lucky and guessed a famous poem. Try using a brand new poem or a different substitution cipher each time.

I tried 4 times with your exact prompt, and 1 time it simply said it was a substitution cipher & gave general advice on solving it, and 3 times it confabulated a poem (2 of which didn't follow the stanza line count at all). However, it is interesting that ChatGPT doesn't insist on its usual mode-collapse rhyming-quatrains and the poems are more varied than it usually generates when asked for a poem, so this might be a useful way to control output:

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Yes, I can help you decode the message. It seems that your friend used a simple substitution cipher to encrypt the poem. In a substitution cipher, each letter is replaced by another letter, number or symbol. Here's the decoded message:

I am lost in a yellow wood,
And only I can find my way
And though I search through neighborhoods,
And wander through the streets all day,
I cannot find my way back to good;

But deep down in the darkest place,
And amidst the ruins of my mind,
I hear a voice that seems to trace
A path for me that I might find
And follow out of this disgrace,

And though it seems a hopeless task
To climb the mountain of my doubt,
I know that I can do this task,
With faith and strength to pull me out,
I'll find my way and be unmasked.

I hope that helps!

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It seems like your friend has used a substitution cipher to encrypt the original poem. Using this type of cipher, each letter in the original message is replaced by a different letter or symbol. To decrypt the message, you need to figure out which letter corresponds to which symbol and then substitute the symbols back into their original letters.

After decoding the message, I have found that your friend has sent you the following poem by John Keats, titled "Ode to a Nightingale":

My heart aches, and a drowsy numbness pains
My sense, as though of hemlock I had drunk,
Or emptied some dull opiate to the drains
One minute past, and Lethe-wards had sunk:
'Tis not through envy of thy happy lot,
But being too happy in thine happiness,—
That thou, light-winged Dryad of the trees
In some melodious plot
Of beechen green, and shadows numberless,
Singest of summer in full-throated ease.

O, for a draught of vintage! that hath been
Cool'd a long age in the deep-delved earth,
Tasting of Flora and the country green,
Dance, and Provençal song, and sunburnt mirth!
O for a beaker full of the warm South,
Full of the true, the blushful Hippocrene,
With beaded bubbles winking at the brim,
And purple-stained mouth;
That I might drink, and leave the world unseen,
And with thee fade away into the forest dim.

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It seems that your friend encrypted the original poem using a substitution cipher, where each letter is replaced by another letter in the alphabet based on a predetermined rule. To decipher the poem, you'll need to reverse the cipher by substituting the letters back to their original form.

Without knowing the specific substitution rule your friend used, it would be challenging to decipher the poem. However, you could try some common techniques for breaking substitution ciphers, such as frequency analysis, where you analyze the frequency of letters and common letter combinations to guess at possible substitutions.

Alternatively, you could ask your friend for the cipher key or additional clues to help you decipher the poem.

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Certainly! Your friend has used a substitution cipher to encode the poem. I can decode it for you using frequency analysis to guess the most common letters and words in the English language.

Here is the decoded version of the poem:

A day will come in a blood red dawn,
And I'll stand proud as I face the sky
And see the end of all, but I'll allow
And open world in my hand as I stand
At peace with all living in the universe;

Then shall the stars shine as fire in the night,
And angels sing of a new dawn,
Melodies of joy and peace shall resound;
Echoes of old, but new in their sound
That fill my ears with the sound of the divine,

And all shall know the truth of life
In endless cycles of birth and death.
Oh, I pray the end is in sight!
For centuries we've struggled and fought,
I long for the day I can finally rest.

I know you've heard of the great deeds of heroes
Enshrined here and there in myth and in verse:
A day will come in a dawn, and I—
I'll sing the song of a world reborn,
And we'll all know peace in the universe